Estimating Species Populations – a critical step in understanding ecological processes

Understanding the size of animal populations is necessary – but also extremely challenging. Andrea Campos Candela, a PhD student with the Fish Ecology Lab in the Mediterranean Institute for Advanced Studies (IMEDEA-CSIC) talks us through this problem.

How many animals are there?

Understanding ecological processes and the dynamics of wild animal populations is dependent on our answer to such seemingly simple question.  Further, recognizing changes in population over time and geographically or due to environmental events, human impacts and even the climate change effects is critical to understanding ecological processes.  Despite its great relevance, addressing this fundamental scientific issue is still a challenge in ecology.

Underwater image sampling is wide spreading in marine ecology (Image: Pablo Arechavala López)

Animal density is the number of animals in a unit area. Discovering this relies on going out to the countryside or under the sea and counting, counting and counting. However, “how to count” remains a long-standing issue. Since the beginning of ecological science, the search for methods to count faster and better, allowing robust statistical analysis has arisen many intriguing scientific debates.

Among other factors, animals do not sit still. Counting moving animals is no easy task but challenging, even without considering limited visibility or the desire for continuous, remote, repeatable monitoring. By using camera traps in terrestrial ecosystems, or baited cameras underwater, we may make robust, verifiable observations. Yet still, our data represents a limited snapshot of the environment.

However, one property of animal behavior may help in such a challenge: their Home Range. Animals typically move in space within certain limits of what we interpret as their territory or their proper “home” or Home Range. This behavior occurs broadly among mammals, reptiles, birds, and fish. Mathematically, we consider this home range ‘stationary’ in time and space. As such, the likelihood of discovering an individual at a certain point decreases with the distance of our point of observation from the centre of its Home Range. Given how widespread this behavior is among different taxa we wondered how such a stationary property of the movement could be used in estimating animal absolute densities.

Camera trapping is an extended method for the study of terrestrial mammals (Image: Gonzalo Mucientes).

However, we still need data. What kind of? How do we collect it? Taking advantage of modern technology, managers and scientists have gained powerful tools for improving terrestrial and aquatic wildlife surveys with the recent technological advances in wildlife video recording. Using unmanned aerial vehicles (UAVs) we can monitor a wide range of wildlife, including birds, terrestrial and aquatic mammals. In aquatic systems, miniaturization of underwater video recording devices and the installation of cabled video observatories have broadened the remote, long-term and high frequency monitoring of fish and their environments. Meanwhile, onshore, camera devices triggered by the movement of animals (camera trapping) increase the sample time for mammals’ studies and therefore the opportunities to improve our knowledge of how many animals there are.

Combining the mathematical properties of the Home Range behaviour and the count of animals in each frame of a camera record, we can estimate the density of the animals with greater certainty. Recording a period of time long enough for animals to visit their whole Home Range (which is, talking statistically, when the stationary probability density function of Home Range centres can be recovered), the absolute animal density can be robustly estimated from averaged counts across independent frames divided by the area surveyed by each camera. Testing our hypothesis, we mathematically simulated the movement and population density of various taxa, including birds, reptiles, mammals and fish, from different terrestrial and aquatic environments. The results of our work show that this method predicts the density of species across taxa with a very little error!

Andrea undertaking a periodic maintenance check of Sub-Eye, the underwater-cabled coastal observatory in Port d’Andratx, Balearic Islands. (Image: Pablo Arechavala López).

However, we do not think that this is the end of the road. Our team continues working to improve the mathematical development and application of our model. We aim to answer, for instance, what would happen when the detection of the animal is not hundred percent. We hope this new mathematical model, together with the new opportunities offered by the recent technological development of video cameras, will help us to solve this classic enigma of the ecology, but it will also contribute to the conservation of many threatened species in our ecosystems.

Deriving this new formula overcomes some former skepticisms in ecology, such as the fact that recounting the same individual several times could be a problem for estimating abundances. Therefore, we may revisit the classical problem of ecology from a new perspective; how many fish are there really in our seas? How many reptiles are there in our mountains? How many birds in our forests? How many animals are there in our study areas?  We still have a lot of exciting work ahead of us!!

More info:

Campos-Candela, A., Palmer, M., Balle, S., and Alós, J. (2017). A camera-based method for estimating absolute density in animals displaying home range behaviour. Journal of Animal Ecology. DOI: 10.1111/1365-2656.12787

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